Project Summary The objective of this NIDDK K01 award is to allow the Candidate to develop a career as an independent, translational investigator in the field of energy metabolism and mitochondrial bioenergetics, through a combination of academic coursework, hands-on training, focused conferences, and implementation of the proposed research project. Specifically, the Candidate will complete several courses on cellular and mitochondrial physiology, bio-imaging (spectroscopy), statistics and bioethics, and will visit the laboratories of two co-mentors in order to apply the academic knowledge to research situations. Under the supervision of the on-site primary and co-mentors, the Candidate will utilize these skills and knowledge to carry out the research objectives of the K01 proposal. In this proposal, the Candidate will utilize a funded study ("Fat Cell Size, Overfeeding &Ectopic Fat";Funding: R01-DK060412-06A1;PI: Ravussin, E) to investigate adaptive thermogenesis in response to 8 weeks of 140% overfeeding in 40 young, healthy males and females. Adaptive thermogenesis is defined as the increase in basal energy expenditure with overfeeding beyond that which is expected based on changes in body mass and composition. Previous overfeeding studies have demonstrated different levels of adaptive thermogenesis, from zero adaptation to significant (>50% of excess energy) adaptation (presented as average adaptation across participants). Additionally, these studies show significant variability in weight gain among subjects, even when the excess calorie burden is equal. Surprisingly few studies have investigated the variability in adaptive thermogenesis in relation to weight gain with overfeeding. We will investigate this relationship in the K01 proposal. Furthermore, we will investigate mechanisms underlying adaptive thermogenesis by measuring mitochondrial bioenergetics (function) before and after overeating, using a novel combination of in vivo spectroscopy and in vitro respiration. Specifically, we will measure changes in mitochondrial uncoupling and resting ATP synthesis (cellular energy demand). Results from this study will fill a critical void in the literature by addressing the question whether adaptive thermogenesis with overfeeding is due to increased skeletal muscle mitochondrial uncoupling, increased cellular energy demand or both. Further studies will investigate the mechanisms responsible for uncoupling and/or increased energy demand, which in the long term, may be important pharmacological targets for energy balance and body weight regulation. PUBLIC HEALTH RELEVANCE: Project Narrative Obesity is on the rise in the United States and worldwide. Weight gain leading to obesity occurs secondarily to positive energy balance, i.e. calorie intake exceeds calorie expenditure. Small differences in energy expenditure may have significant effects on body weight regulation over time. Adaptive thermogenesis with excess calories (overfeeding), or the ability to increase energy expenditure at rest out of proportion to the increase in body size, may be an important mechanism by which excess energy is dissipated to regulate body weight. However, the contribution of adaptive thermogenesis to weight gain with overfeeding is unclear. Additionally, the mechanisms underlying adaptive thermogenesis, including skeletal muscle bioenergetics, are presently unknown.